Numerous test methods exist for characterizing adhesives and bonded joints, and may be used to determine fatigue resistance, environmental durability and creep behavior. Adhesive tests can be divided into those methods that provide mechanical property data for the adhesive, which aids the selection of adhesives, and those methods that can be used to determine the quality of adhesively bonded structures, and thus aid the design process of adhesive joints.
An ASTM F88/F88M-09 test method is a standard test method for seal strength of flexible barrier materials, which covers the measurement of the strength of seals in flexible barrier materials. The test may also be conducted on seals between a flexible material and a rigid material. This test method measures the force required to separate a test strip of material containing the seal. It also identifies the mode of specimen failure, and seals tested in accordance with this test method may be from any source, laboratory or commercial.
Seal strength is a quantitative measure for use in process validation, process control, and capability. Seal strength is not only relevant to opening force and packaging integrity, but to measuring the packaging processes' ability to produce consistent seals. Seal strength at some minimum level is a necessary packaging requirement, and at times it is desirable to limit the strength of the seal to facilitate opening.
One of the testing machines is of the constant rate-of-jaw-separation type. The machine shall be equipped with a weighing system that moves a maximum distance of 2% of the specimen extension within the range being measured. The machine shall be equipped with a device for recording the tensile load and the amount of separation of the grips. It is noted that both of these measuring systems shall be accurate to ±2%. The rate of separation of the jaws shall be uniform and capable of adjustment from approximately 8 to 12 inches [200 to 300 mm]/min. The gripping system shall be capable of minimizing specimen slippage and applying an even stress distribution to the specimen. It is noted that if calculation of average seal strength is required, the testing machine system shall have the capability to calculate its value over a specific range of grip travel programmable by the operator. Preferably, the machine shall have the capability also to plot the curve of force versus grip travel.
Among the test methods used to determine the seal strength of an adhesive including peel, shear, cleavage, and tension tests, peel tests are common for tapes, labels, coatings, and other bonded materials, which measure the force required to separate a test strip of material containing the seal. If the test strip peels apart in the seal area, either by adhesive failure, or cohesive failure, the average peel force is an important index of performance of the performance of the seal.
While performing the test, the tensile testing machine has to be calibrated. The test specimens are then prepared by cutting to the dimensions in accordance with the test method, such as shown in FIG. 1. It is noted that edges should be clean-cut and perpendicular to the direction of the seal. Specimen wings may be shorter than shown, depending on the grip dimension of the testing machine.
The test specimen is then transferred to the testing machine by clamping each wing of the test specimen in the testing machine. The sealed area of the specimen shall be appropriately equidistant between the grips. The specimen should also be centered laterally and aligned in the grips, so the seal line is perpendicular to the direction of pull, allowing sufficient slack so the seal is not stressed prior to initiation of the test.
According to the test results, the orientation of a fin-seal tail during the test is a major factor to affect the measurement of the seal strength. It is critical that the specimen is precisely positioned in the grips of the testing machine because the testing results can vary significantly if the specimen is not properly and precisely positioned in the grips. Operators may manually load the specimens into a tensile testing machine and visually check the orientations and positions of the specimens in the grips, which may usually lead to errors due to improper or inconsistent loading operations, and the testing results may be adversely affected. Therefore, there remains a need for a new and improved method and apparatus for consistent and precise loading of the specimens.